1. Field of the Invention
The present invention relates generally to semiconductor materials. More specifically, the present invention relates to growth of ferrite materials on a semiconductor substrate.
2. Description of the Related Art
The next generation of radar electronics employs simultaneous transmit and receive (STAR) technology incorporating magnetic ceramics in multiport devices allowing simultaneous transmission of electromagnetic signals. The applications of this technology are within radar systems that enable continuous tracking of multiple targets. This technology is also applicable in the wireless communications market as well, cellular, and satellite communications.
An important component to radar electronics is the circulator. The operation of a circulator is based on the manipulation of electromagnetic (EM) signals in a multiport nonreciprocal device. For example, the signal entering into the first port is wholly coupled to an adjacent port while the EM signal is isolated from the other port(s). Circulators with three or more ports have nonreciprocal properties that are highly valued in many microwave systems. Magnetic ceramics, such as ferrites, that are biased in a DC magnetic field exhibit anisotropic wave propagation characteristics. These characteristics of the EM waves gives rise to a non-reciprocal rotation of the plane of polarization of the EM wave, a non-reciprocal phase shift and a non-reciprocal displacement of the microwave field pattern. Ferrites (e.g., magnetic oxides that have both desirable dielectric and magnetic properties and, most importantly, are insulators) have been an integral part of nearly all circulator device designs that operate at μ- and mm-wave frequencies.
Although spinel ferrites and garnets have been used in many applications from 1-10 GHz, the hexaferrites with appropriate cation substitutions have proven to be versatile for applications from 1-100 GHz. Ba-hexaferrite (BaM) is the prototypical material that is considered to be an enabling material for such applications. Accordingly, it would be desirable to employ hexaferrites in circulators, isolators, filters, and phase shifter applications.
In addition, one long standing problem facing the ferrite materials and device communities is the need for integration of these ferrite devices with semiconductor platforms (i.e. CMOS systems). In order to make this a reality one needs to process ferrites on semiconductor substrates. High crystal quality, low microwave loss, ferrites are processed at elevated temperatures typically approximately 900° C.; temperatures at which traditional semiconductor materials degrade. This has been the limiting factor.
The deposition of ferrites onto a semiconductor substrate is currently not possible due to the mismatch in lattice parameters between BaM and SiC, for example. In other words, the atoms within the ferrite (BaM) layer and the semiconductor (SiC) layer are unable to properly align causing strains and poor crystal alignment. In microwave applications, for example, this mismatch leads to unacceptable losses.
In recent years new wide bandgap semiconductor materials (i.e., SiC and GaN) have been developed that are stable to very high temperatures. With a high saturated drift velocity (˜2.7*107 cm/s), wide bandgap (3.03 eV), and high breakdown electric field strength (2.4×106 V/cm), SiC holds unique potential as a substrate for the next generation of high-temperature, high-frequency, high-power electronics. In contrast to other substrates being used in ICs and MMICs (monolithic microwave integrated circuits), the strong covalent Si—C bond provides this material with high thermal stability and chemical inertness. Furthermore, the atomic lattice mismatch between SiC and BaM is less than 5% allowing for their possible integration.